Numerous processes for the production of olefins, such as steam cracking, catalytic cracking and viscoreduction produce fractions polluted by molecules more unsaturated than the sought olefins. The satisfactory use of these fractions for producing finished products involves the elimination of these molecules which contain conjugate double bonds and/or triple bonds. The selective hydrogenation of the latter into the corresponding olefins is the preferred process for removing these, while still recovering the sought olefins.
These hydrogenation reactions are generally performed in a temperature range between 20.degree. and 200.degree. C. under a pressure between 10 and 100 bar (1 and 10 megapascal) and with a space velocity between 1 and 40 m.sup.3 /m.sup.3 of catalyst/h. The catalysts generally used are constituted by one or more metals deposited on an oxide support. The preferred base metals are those of group VIII and more particularly .nickel, palladium and platinum. The supports are often chosen among alumina, silica, silica-aluminas, aluminates or charcoal.
The industrial use of such catalysts often takes place in the presence of additives for the purpose of improving the selectivity of the hydrogenation reaction. The most widely used compound is carbon monoxide, as claimed in EP-81,041.
The development of catalysts with better performance characteristics from the activity and selectivity standpoints has led to the introduction of other metals into the catalytic formulations. Reference can e.g. be made to silver (U.S. Pat. No. 4,409,410) and gold (U.S. Pat. No. 4,490,481), which very significantly improve the catalytic properties of group VIII metals for the hydrogenation reaction.